EDMs from the QCD θ term

Transcription

1 ACFI EDM School November 2016 EDMs from the QCD θ term Vincenzo Cirigliano Los Alamos National Laboratory 1

2 Lecture II outline The QCD θ term Toolbox: chiral symmetries and their breaking Estimate of the neutron EDMs from θ term The Strong CP problem: understanding the smallness of θ Peccei-Quinn mechanism and axions Induced θ term 2

3 The QCD θ term 3

4 The θ term The QCD Lagrangian contains in principle the following term: ε μναβ = 4-dim Levi-Civita symbol gs = strong coupling constant 4

5 The θ term The QCD Lagrangian contains in principle the following term: ε μναβ = 4-dim Levi-Civita symbol gs = strong coupling constant Multiple reasons for the presence of θ term: EFT perspective: at dimension=4, include all terms built out of quarks and gluons that respect SU(3)C gauge invariance Diagonalization of quark mass matrix mq induces Δθ = arg det mq (will discuss this later) Structure of QCD vacuum (won t discuss this) 4

6 The θ term The QCD Lagrangian contains in principle the following term: ε μναβ = 4-dim Levi-Civita symbol gs = strong coupling constant Transformation properties under discrete symmetries: analogy with Electrodynamics E is P-odd, T-even B is P-even, T-odd P-even, T-even P-odd, T-odd 5

7 The θ term The QCD Lagrangian contains in principle the following term: ε μναβ = 4-dim Levi-Civita symbol gs = strong coupling constant θ term is P-odd and T-odd, and hence CP-odd (CPT theorem) How do hadronic CP-violating observables depend on θ? (After all, no breaking of P and T observed in strong interactions) 6

8 Toolbox: chiral symmetries and their breaking Relevant to understand 1. How to compute the neutron EDM from the θ term 2. How the Peccei-Quinn mechanism works Technical subject: I will present the main concepts and implications 7

9 Chiral symmetry 8

10 Chiral symmetry L,R U(3) For mq = 0, action invariant under independent U(3) transformations of left- and right-handed quarks: Conserved vector and axial currents (T a : SU(3) generators and identity) 8

11 Chiral symmetry L,R U(3) For mq = 0, action invariant under independent U(3) transformations of left- and right-handed quarks: Symmetry is broken by mq 0 and by more subtle effects 9

12 Symmetry breaking In general, three known mechanisms for symmetry breaking Explicit symmetry breaking Symmetry is approximate; still very useful Spontaneous symmetry breaking Equations of motion invariant, but ground state is not Anomalous (quantum mechanical) symmetry breaking Classical invariance but no symmetry at QM level 10

13 Symmetry breaking In general, three known mechanisms for symmetry breaking Explicit symmetry breaking Symmetry is approximate; still very useful Spontaneous symmetry breaking Equations of motion invariant, but ground state is not Anomalous (quantum mechanical) symmetry breaking Classical invariance but no symmetry at QM level All relevant to the discussion of chiral symmetry in QCD and Peccei-Quinn symmetry 10

14 Spontaneous symmetry breaking Action is invariant, but ground state is not! Continuous symmetry: degenerate physically equivalent minima Excitations along the valley of minima massless states in the spectrum (Goldstone Bosons) Many examples of Goldstone bosons in physics: phonons in solids (translations); spin waves in magnets (rotations); 11

15 Spontaneous symmetry breaking Pions, kaons, mesons: Goldstone bosons associated with SSB of chiral symmetry Figure from M. Creutz, Axial subgroup is broken. Vector subgroup SU(3)V stays unbroken (symmetry approximately manifest in the QCD spectrum) In case of SSB currents are still conserved. Massless states appear in the spectrum. What about the U(1)A symmetry? 12

16 Anomalous symmetry breaking Action is invariant, but path-integral measure is not! 13

17 Anomalous symmetry breaking Action is invariant, but path-integral measure is not! Chiral anomaly [U(1)A]: in mq=0 limit axial current not conserved Axial transformation induces a shift in the θ term 13

18 Anomalous symmetry breaking Action is invariant, but path-integral measure is not! Chiral anomaly [U(1)A]: in mq=0 limit axial current not conserved Axial transformation induces a shift in the θ term 13

19 Implications for θ term Diagonalization of quark mass matrix mq induces Δθ = arg det mq Diagonal mq matrix has complex eigenvalues To make them real, additional axial rotation is needed This induces shift in θ proportional to 14

20 Implications for θ term Diagonalization of quark mass matrix mq induces Δθ = arg det mq Diagonal mq matrix has complex eigenvalues To make them real, additional axial rotation is needed This induces shift in θ proportional to Physics depends only on the combination Can put it in the gluonic θ term or in a complex quark mass! 14

21 Estimate of the neutron EDM from θ term Crewther, Di Vecchia, Veneziano, Witten Phys. Lett. 88B, 123 (1979) 15

22 Rotating CPV to quark mass In order to analyze pion-nucleon couplings, it is more convenient to put the strong CPV in the form of pseudoscalar quark densities 16

23 Rotating CPV to quark mass In order to analyze pion-nucleon couplings, it is more convenient to put the strong CPV in the form of pseudoscalar quark densities Use freedom in SU(3)A transformation to ensure that perturbation introduces no mixing of the vacuum to Goldstone Bosons ( Vacuum alignment ) 16

24 Rotating CPV to quark mass This requires A to be proportional to the identity, with Effect disappears if one of the quark masses vanishes 17

25 CPV pion-nucleon coupling Use chiral symmetry (soft pion theorem) to relate CPV pionnucleon coupling to baryon mass splittings Crewther-DiVecchia- Veneziano-Witten

26 CPV pion-nucleon coupling Use chiral symmetry (soft pion theorem) to relate CPV pionnucleon coupling to baryon mass splittings Crewther-DiVecchia- Veneziano-Witten 1979 Equivalent way to see this: θ and mass splitting are chiral partners. Low-energy couplings controlling the two are related 18

27 CPV pion-nucleon coupling Use chiral symmetry (soft pion theorem) to relate CPV pionnucleon coupling to baryon mass splittings Crewther-DiVecchia- Veneziano-Witten 1979 Mereghetti, van Kolck and refs therein 19

28 Chiral loop and estimate of dn Leading contribution (for mq 0) to neutron EDM via chiral loop Crewther-DiVecchia- Veneziano-Witten 1979 E. Mereghetti et al Phys. Lett. B 696 (2011) 97 Counter-term (of same order) and subleading contributions 20

29 Chiral loop and estimate of dn Leading contribution (for mq 0) to neutron EDM via chiral loop Crewther-DiVecchia- Veneziano-Witten 1979 E. Mereghetti et al Phys. Lett. B 696 (2011) 97 Counter-term (of same order) and subleading contributions 20

30 Chiral loop and estimate of dn Leading contribution (for mq 0) to neutron EDM via chiral loop Crewther-DiVecchia- Veneziano-Witten 1979 E. Mereghetti et al Phys. Lett. B 696 (2011) 97 Counter-term (of same order) and subleading contributions Recent lattice QCD results** do not change qualitative picture Guo et al., Akan et al., Alexandrou et al.,

31 Chiral loop and estimate of dn Leading contribution (for mq 0) to neutron EDM via chiral loop Crewther-DiVecchia- Veneziano-Witten 1979 E. Mereghetti et al Phys. Lett. B 696 (2011) 97 Counter-term (of same order) and subleading contributions Recent lattice QCD results** do not change qualitative picture 21

32 The strong CP problem: understanding the smallness of θ _ 22

33 Understanding the smallness of θ The small value of begs for an explanation Possible ways out: One of the quark masses vanishes (so can rotate away θ): this is strongly disfavored by phenomenology of light quark masses** Invoke some symmetry principle P or CP exact at high scale, broken spontaneously at lower scale. Difficulty: keep θ<10-10 while allowing large CKM phase Peccei-Quinn scenarios ** See Wilczek-Moore 1[ ] for a reincarnation of this idea through cryptoquarks": massless quarks confined in super-heavy bound states 23

34 Peccei-Quinn mechanism _ Basic idea: promote θ to a field and make sure that it dynamically relaxes to zero How to get there: extend the SM with additional fields so that the model has an axial U(1)PQ global symmetry with these features: U(1)PQ is broken spontaneously at some high scale axion is the resulting Goldstone mode U(1)PQ is broken by the axial anomaly the axion acquires interactions with gluons, which generate an axion potential _ Potential induces axion expectation value such that θ=0 Salient features can be captured by effective theory analysis 24

35 Axion effective theory At energies below the U(1)PQ breaking scale fa, axion effective Lagrangian is given by We can ignore derivative terms irrelevant for strong CP problem, such as Goldstone nature of the axion requires the effective Lagrangian to be invariant under a(x) a(x) + constant ** (up to the anomaly term) The presence of this term is required by the axial anomaly ** In simplest models, the axion is the phase of a complex scalar charge under U(1)PQ Hence the transformation property 25

36 Axion effective theory At energies below the U(1)PQ breaking scale fa, axion effective Lagrangian is given by We can ignore derivative terms irrelevant for strong CP problem, such as Goldstone nature of the axion requires the effective Lagrangian to be invariant under a(x) a(x) + constant (up to the anomaly term) The presence of this term is required by the axial anomaly Key point: in LQCD +La, a(x) leads to a field-dependent shift of θ 26 Through interactions with gluons this quantity acquires a potential

37 Axion effective theory In absence of other sources of CP violation, the potential is an even function of 27

38 Axion effective theory In absence of other sources of CP violation, the potential is an even function of Minimum of the potential when vanishes. This solves the strong CP problem, independently of the initial value of θ 28

39 Axion effective theory In absence of other sources of CP violation, the potential is an even function of Minimum of the potential when vanishes. This solves the strong CP problem, independently of the initial value of θ Axion mass given by with 28

40 Induced θ term In presence of other sources of CP violation beyond the θ term, the potential is not an even function: 29

41 Induced θ term In presence of other sources of CP violation beyond the θ term, the potential is not an even function: This needs to be taken into account when computing the impact of BSM operators on EDMs 29

42 Status of axion searches Axion as cold dark matter lives here Disfavored by astrophysics / cosmological observations (grey) or argument (blue) Sensitivity of planned experiments 30 ArXiv:

43 Backup slides 31

44 Abelian gauge theory Recall U(1) (abelian) example Form of the interaction: conserved current associated with global U(1) 32

45 Non-abelian gauge theory Generalize to non-abelian group G (e.g. SU(2), SU(3), ). Invariant dynamics if introduce new vector fields transforming as 33

46 Anomalous breaking of B and L Action is invariant, but path-integral measure is not! 34

47 Anomalous breaking of B and L Action is invariant, but path-integral measure is not! Baryon (B) and Lepton (L) number are anomalous in the SM Only B-L is conserved; B+L is violated; negligible at zero temperature 34

48 θ term and topology θ term is total derivative (surface term) but can t ignore it due to non-trivial topological effects Difference in winding number of gauge fields t = ± 35

49 CP and chiral symmetry Chiral symmetry (ΨL,R e ±χ ΨL,R) is spontaneously broken Figure from M. Creutz, Degenerate vacua. Each spontaneously breaks all but one CP χ = χ -1 CPχ Choice of fermion phases: CP0 (standard CP) is preserved ( iψγ5ψ Ω) = 0 ) This defines a reference vacuum Ω 36

50 CP and chiral symmetry Chiral symmetry (ΨL,R e ±χ ΨL,R) is spontaneously broken Chiral symmetry is explicitly broken by quark masses and BSM operators Figure from M. Creutz, Degenerate vacua. Each spontaneously breaks all but one CP χ = χ -1 CPχ Choice of fermion phases: CP0 (standard CP) is preserved ( iψγ5ψ Ω) = 0 ) This defines a reference vacuum Ω Explicit chiral symmetry breaking δl lifts degeneracy, i.e. selects true vacuum and the associated unbroken CP If we want true vacuum to be Ω then δl cannot be arbitrary. It satisfies 36 Vacuum alignment

51 Chiral symmetry relations Prototype: theta term and mass splitting are chiral partners Nucleon matrix elements are related. At LO (soft pion theorem) Crewther-DiVecchia- Veneziano-Witten 1979 (with LQCD input) Corrections appear at NNLO, not log enhanced Mereghetti, van Kolck and refs therein 37

52 Toy model of invisible axion Shifman-Vainshtein-Zakharov Nucl. Phys. B 166 (1980) 493 Field content: new quark (only strong interactions) + New complex scalar Yukawa interactions invariant under axial U(1)PQ φ acquires VEV Quark and radial scalar excitations super-heavy. Axion is identified the phase of the scalar field: Super-heavy quarks mediates axion-gluon interaction via triangle diagram: From this point on, the analysis proceeds as in the EFT description 38